Schweitzer Engineering Laboratories SEL-421-4, Instruction Manual

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6.164
SEL-421 Relay Instruction Manual Date Code 20171021
Protection Applications Examples
Circuit Breaker Failure Application Examples
To maintain system stability, you must clear the fault within the total clearing
time. Use the maximum operating time of the local and remote circuit breakers.
The maximum operating time of the circuit breaker, t
l-bk
, is 2 cycles for this
example. Also use the maximum reset time of 50FA1; the maximum reset (drop-
out) time of the phase current level detector, t
50r
, is 1 cycle. You must also
include the communications channel time, t
ch
, for remote circuit breaker tripping.
To determine setting BFPU1, you must find the safety margin, t
s
. Determine the
safety margin from Figure 6.40 .
Equation 6.93
Use the safety margin result from Equation 6.94 to calculate BFPU1:
Equation 6.94
BFPU1 := 10.000 .
Breaker Failure Time Delay—BK1 (0.000–6000 cycles)
Figure 6.47 Timing Diagram for Setting BFPU1—Scheme 2
Breaker
Operate Time t
l-bk
BFPU1 SPBFPU1
Line Protection
Maximum Operate Time
Fault Occurs
50FA1
BFIA1
FBF1
50FA1
Dropout Time, t
50r
Safety
Margin Time, t
s
where:
t
s
= safety margin
t
t
= total clearing time (15 cycles)
t
lr
= line protection maximum operating time (2 cycles)
t
l-bk
= local circuit breaker maximum operating time (2 cycles)
t
50r
= circuit breaker failure overcurrent element 50FA1 maximum
reset time (1 cycle)
t
86
= auxiliary breaker failure relay operating time (1 cycle)
t
ch
= communications channel maximum operating time (1 cycle)
t
r-bk
= remote circuit breaker maximum operating time (2 cycles)
t
s
t
t
t
1r
t
1 bk
–
t
50r
t
86
t
ch
t
r bk
–
+ + + + +
 
– =
15 2 2 1 1 1 2
+ + + + +
 
– =
6 cycles
=
BFPU1 t
1 bk
–
t
50r
t
s
+ + =
3 1 6
+ + =
10 cycles
=